1. Field of the Invention
This invention relates to the processing of periodic information signals, such as, video signals, and more particularly is directed to apparatus by which time base errors introduced during recording and/or reproducing of such signals may be removed.
2. Desription of the Prior Art
Video signals are frequently recorded on magnetic tape and subsequently reproduced for later broadcasting or viewing purposes. During the reproduction of recorded video signals, time base or frequency errors are usually introduced by reason of expansion or contraction of the record medium during or after recording, variation in the speed of the tape relative to the magnetic head or heads during recording or reproduction, variation between the tape recording speed and the tape reproducing speed, and the like. Such time base errors, when present in the reproduced video signals, cause a frequency shift of the latter which can result in many observable undesirable effects, particularly when the reproduced video signals are to be transmitted or broadcast and may be mixed with live broadcast material that do not have such time base errors. The observable undesirable effects resulting from relatively small time base errors are a smeared or jittery picture with erroneous intensity variations and, in the case of color video signals, improper color display. When the time base errors are large, the reproduced picture will fail to lock horizontally or vertically.
In an existing time base corrector for substantially removing time base errors from video signals, for example, as disclosed in U.S. Pat. No. 3,860,592, issued Jan. 14, 1975, the incoming video signals are converted from analog to digital form and temporarily stored in a memory. Time base errors are removed from the video signals by writing the digitized signals in the memory at a clocking rate which varies in a manner generally proportional to the time base errors, and by fetching or reading out these stored signals at a standard clocking rate. After such reading out of the digitized video signals, the latter are reconverted to analog form and applied to an output terminal. The memory used in the known time base corrector comprises a plurality of memory units each capable of storing one or more horizontal lines of video information. A sequence control unit controls the selection of each memory unit for writing and reading so that the sample video information is sequentially stored by cyclically enabling the plurality of memory units and serially storing one or more lines of digitized video information on in each selected memory unit, and further so that, contemporaneously with the storage of sampled video information in a selected memory unit, the sequence control unit enables the video information stored in a different one of the memory units to be sequentially fetched or read out therefrom, with the enabling of the memory units for the reading out of the information stored therein being also effected in a cyclical manner. However, the arrangement disclosed in the above identified patent for preventing double clocking of a single memory unit, that is, an attempt to read and write contemporaneously from the same memory unit in response to an excessive time base error, results in at least one incomplete or deteriorated line interval signal, and possibly even two incomplete or deteriorated line interval signals which are out of horizontal synchronization with each other and which are present in the output from the time base corrector. Further, the above referred to existing time base corrector is not capable of eliminating from its output those line intervals of the incoming video signals in which crop-outs may occur.
In view of the above, it has been proposed, for example, in U.S. Patent Application Ser. No. 642,197, filed Dec. 18, 1975, and having a common assignee herewith, to provide a time base corrector generally of the type described above and in which those line intervals of the incoming video signals having drop-outs occurring therein are omitted from the output of the time base corrector and replaced by previously stored line intervals of similar video information. In such time base corrector, the elimination of video signals containing drop-outs is achieved merely be extending the writing period of a memory unit in response to a detected drop-out in the incoming video signals so as to store, in such memory unit, the next occurring line interval which is free of drop-out, and, thereafter, during reading out of the stored signals, the line interval preceding the detected or omitted line interval is read twice to replace the omitted line interval. The foregoing arrangement is generally satisfactory except in the case where drop-outs occur in two or more successive line intervals of the incoming video signals, in which case the line interval preceding the onset of drop-out is repeated three or more times in the output of the time base corrector and such repetition of a single line interval may be perceptible in the picture reproduced from the corrected video signals. Moreover, in order to avoid double-clocking of a memory unit in response to excessive time base errors in the incoming video signals, the writing or reading period of a memory unit is extended, for example, from a normal one line interval to two line intervals, and such concept for avoiding double-clocking may accentuate the above problem associated with the elimination of drop-out.
Further, in the existing time base correctors, as described above, the read out of the temporarily stored digitized video signals is effected at a fixed, standard clocking rate, and thus cannot compensate for velocity or phase errors occurring within a line interval of the incoming video signals.